This is one of four packages forcing old mock: >=dev-python/mock-1.0.1[${PYTHON_USEDEP}] <dev-python/mock-1.1[${PYTHON_USEDEP}] Please look into making it compatible with modern versions of packages.
looks like upstream removed the cap when they dropped testing on python-2.6, so fixed in 2.4.2.0-r1 and 9999.